Congestion management refers to the measures taken by transmission system operators (TSOs) to prevent overloads on nationwide power lines, so-called grid bottlenecks. Congestions are caused by excess electricity production, which cannot be removed due to a lack of capacity on the physical power lines. In order to avoid such congestion situations, congestion management measures are used, the most important of which is the so-called “redispatch”. This is a nationwide option to control all large generation plants and to throttle or start up as required. If the TSOs predict a congestion, generation plants are throttled before the congestion and started up behind the bottleneck. As a result, the absolute production of electricity does not change; only the place of production is changed in order to relieve the load on the power line and thus avoid the congestion. In 2021, a total of around 21 TWh (10.5 TWh feed-in reductions & 10.5 TWh feed-in increases) were regulated as part of feed-in management, which resulted in costs of around 2.3 billion euros. (Network Congestion Management Report 2021, BNetzA) With the help of redispatch, the operating times of nationwide generation plants are therefore interfered with in order to avoid congestions that occur in the network.
Just like generation plants, storage systems such as large-scale battery storage systems must also be available for the redispatch measures of the TSOs. If they are strategically placed at grid nodes where grid congestion frequently occurs, they can even make a particularly effective contribution to avoiding congestion. In addition to normal redispatch measures, i.e. throttling the output of the large-scale battery storage systems, the storage systems can also function as consumers. In this way, large-scale battery storage systems can not only be throttled to zero before bottlenecks like generation plants, but can also absorb surplus electricity. Once the bottleneck has been avoided, the storage systems can feed this electricity back into the grid with a time delay.With a large-scale expansion of large-scale battery storage systems, which both the BNetzA and the Fraunhofer Institute are forecasting, this potentially more effective Redispatch 3.0 can greatly relieve the nationwide electricity grid. In this way, large-scale battery storage systems can not only avoid bottlenecks, but also minimize the shutdown of renewables such as wind turbines in the north and limit grid expansion. Furthermore, the costs of congestion management measures are drastically reduced by avoiding shutdowns with the help of storage systems, which can reduce our grid fees.
The reason for the congestion is that the current electricity infrastructure in Germany is not designed for the high volatility of renewable generation plants. As a result of severe weather dependency, the electricity production of renewable sources fluctuates strongly. However, the power grid has historically been designed for very constant, conventional large power plants and is only slowly adapting to rapidly changing electricity production. In the past, the power grid was often referred to as a nationwide “copper plate,” which describes a power grid that can conduct the electricity produced in all directions at any time. The fact that this description no longer corresponds to reality can be seen from the well-known North-South congestion. As a result of the relatively rapid expansion of onshore and offshore wind turbines in northern Germany, a considerable surplus of electrical energy is produced there on windy and sunny days. In order not to overload the network, this surplus must be transported south of Germany to major consumption centers such as industrial sites and major cities. Since physical network expansion, i.e. the expansion of electricity grid capacity, is significantly lagging behind the expansion of renewable sources, North-South congestions often occur. The result is a shutdown of renewable generation plants through redispatch and a ramp-up of conventional power plants in the south. 3% of the total renewable production capacity was shut down as early as 2021. Up to 2 million households could have been supplied with this amount of electricity. With the federal government's ambitious expansion goals of 80% renewable electricity by 2030, grid bottlenecks will occur even more frequently. From 2020 to 2021 alone, the volume of congestion management measures implemented rose by 19%. (Network Congestion Management Report 2021, BNetzA) In today's reality, the German power grid can no longer be described as a nationwide “copper plate”. However, such a system is only desired to a limited extent in a renewable future. Due to the extremely high fluctuations in renewables, a significant surplus of electricity is produced at peak times. Designing the power grid for this peak kWh production so that all electricity could be transported without congestions even at absolute maximum times would be extremely uneconomical and would cost many billions of euros. An alternative to the resulting redispatch measures, which result in the shutdown of renewables, is the large-scale integration of storage systems, such as large battery storage or pumped storage. These would both limit network expansion to a realistic level and minimize redispatch measures.
All redispatch measures are carried out and are responsible for the 4 transmission system operators. In the course of the previous day, the TSOs receive both the timetables of all generation plants and the estimated consumption for the following day. With this overview, possible grid congestions in the entire nationwide power grid are predicted through regional load flow calculations. Based on this data, the TSOs determine the redispatch measures for the following day and, if necessary, correct them over the course of the day if short-term fluctuations occur. For these measures, all generation plants with an installed capacity of 100 kW or more must be available. This limit was put into effect with the introduction of Redispatch 2.0 on 01.10.2021. Previously, the limit was 10 MW, meaning that only large plants took part in the redispatch. As a result of the large expansion of smaller renewable generation plants, this limit has been adjusted so that the TSOs have adequate opportunities to intervene in an energy system dominated by renewable energies. In addition, Redispatch 2.0 focused on shutting down conventional systems. For example, renewable plants are only switched off when conventional options have been exhausted or the shutdown of renewable energy is cheaper by a factor of 10.
If generation plants are instructed by the TSOs to reduce or increase their output, this will invariably result in additional costs for the producer. These additional costs are covered by the TSOs, which in turn transfer the costs to all consumers in the form of network charges. These network charges are included in the electricity prices paid.
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